The present invention refers to a method for determining the configuration of detectors of a danger alarm system of the type having a central station which is parallel connected to the detectors via a two-wire communication line in form of a loop and/or stubs, with each detector including i.e. a microprocessor by which a current drain is controlled for data exchange with the central station by means of current pulses and an address register.
European publication EP-A1-0 191 239 discloses a danger alarm system with detectors which are parallel connected in a two-wire communication line and include particular structural features by which the central station is able to recognize the installation sequence of the detectors. The recognition is carried out regardless as to whether the communication line is a stub, a loop or a combination of both. Each detector has at least one relay, with the communication line running across the contacts of the relay. Further, each detector includes an address register and a microprocessor which allows a data exchange with the central station. During initial breaking in of the danger alarm system, the so-called initialization routine, the relay contacts are open in all detectors. The central station assigns to the first, i.e. the nearest detector, an address and transmits to this detector the command to store this address and to activate its relay for closing its contacts. In a like manner, the central station communicates with the second detector and the following detectors. After terminating the initialization routine, the central station has individually recognized all detectors and is able to communicate with them via their address if the communication line is a simple stub or loop. In the event, the installation includes several, possibly further branched stubs and/or subloops, special detectors are installed at the branch-off points or junction points, with the special detectors containing a second relay which operates with the first relay as a so called T-switch. In this case, the initialization routine is initially done in direction towards the branch ends (stub or subloop) until reaching the pertaining last detector. The central station then continues from the branch-off point in the other branch-off direction after transmitting to the respective detector the command for switching over its T-switch. Through recognition of the sequence of the detectors and the position of the particular, T-switch containing detectors, the topology of the system, i.e. the precise configuration of its detectors can be determined.
A danger alarm system of this type has a drawback that in order to attain a desired small power consumption, this system requires the equipment of each detector with an expensive bistable relay. This drawback is compounded by the fact that those special detectors which are located at the branch-off points require two such relays. Substitution of such relays through semiconductor circuits is not possible because the serial connection results in increasing voltage drops and apart from that, would also not result in a more cost-effective system.
In this conventional system, the address assigned to a detector designates also the location of installation of the detector so that an exchange of two or more detectors which is not recognized by the central station would result in a misdirection of e.g. intervening forces because alarm signals triggered by these detectors would be interpreted as being originating from the respective original location of installation. In order to prevent such errors, the known system stores the detector address in a volatile memory which means that this information is lost when removing the detector. Moreover, the removal of more than one detector is indicated in the central station as malfunction so that a correction of the malfunction has to be followed by a new initialization. Even though the described problem could be eliminated in a system in which the address register of each detector is located in its generally fixedly secured pedestal; the necessity of a second printed circuit in each detector pedestal as well as respective junction contacts to the detectors would result in prohibitively high costs and less reliable operation so that such a solution is not feasible.